An arithmetic series is such that the 10th term is 40 and the sum of the first 10 terms is 265. Find the sum of the first 20 terms.

Answer 1

T10 = a+9d = 40
S10 = 10/2 (a+40) = 265
a = 13
so, d=3

S20 = 20/2 (2*13+19*3) = 830